1. Field of the Invention
The present invention relates to a liquid crystal display device for use as a display section of a notebook personal computer, a portable terminal apparatus, and the like.
2. Description of the Related Art
FIG. 9 is a circuit diagram illustrating a configuration of a conventional liquid crystal display device 10.
The liquid crystal display device 10 includes a plurality of switching elements (TFTs 2 in this example) which are arranged in a matrix pattern on an active matrix substrate (TFT substrate). The liquid crystal display device 10 also includes gate signal lines 3 for supplying gate signals for driving the TFTs 2 and source signal lines 4 for supplying display signals (source signals) to the TFTs 2. The gate signal lines 3 and the source signal lines 4 are arranged so as to cross each other. The gate electrode of each TFT 2 is electrically connected to the gate signal line 3, the source electrode of each TFT 2 is electrically connected to the source signal line 4, and the drain electrode of each TFT 2 is connected to a pixel electrode 1 and to one electrode of an auxiliary capacitor (Cs) 5. The other electrode of the auxiliary capacitor (Cs) 5 is connected to a common line 6. The TFT substrate opposes a counter substrate (color filter (CF) substrate) with a liquid crystal layer being interposed therebetween.
The liquid crystal display device 10 is driven, for example, by scanning the gate signal lines 3 upwardly or downwardly to turn ON the TFTs 2 along each gate signal line 3. A source signal is applied to each pixel (across the liquid crystal layer in that pixel) so as to charge the liquid crystal layer and the auxiliary capacitor 5 of that pixel to the potential of the source signal, whereby the potential of the liquid crystal layer in each pixel is kept constant after the TFT 2 is turned OFF until the pixel is scanned in the next sequence. Thus, an image is displayed on the liquid crystal display device 10.
When the liquid crystal material of the liquid crystal display device 10 is contaminated with an ionic impurity, some current is conducted through the liquid crystal layer before the next sequence so as to reduce the potential which has been applied across the liquid crystal layer. In such a case, a normal display cannot be maintained.
Such an ionic impurity may be any organic and inorganic impurity, e.g., Na+, Ca2+, Cu2+, Clxe2x88x92, OHxe2x88x92, COOHxe2x88x92, or the like. Such an ionic impurity may easily be introduced into the liquid crystal material during the production process of the liquid crystal display device.
In recent years, liquid crystal display devices have been used in portable terminal apparatuses. Therefore, attempts have been made in the art to reduce the power consumption of the liquid crystal display devices so that the portable terminal apparatuses can be used outdoor for a long period of time. Accordingly, it has been necessary to develop a liquid crystal material which can be driven with a low voltage. However, the capability of being driven with a low voltage means that the liquid crystal material has a large dielectric anisotropy, which in turn means that the liquid crystal material itself has a potential. Such a liquid crystal material itself is likely to attract an ionic substance, thereby increasing the probability that the liquid crystal material may be contaminated during the production process of the liquid crystal display device.
It is well known in the art that increasing the auxiliary capacitance Cs is effective to address these problems. However, increasing the auxiliary capacitance Cs has a problem of reducing the aperture area of each pixel. Then, in order to achieve a display brightness of a liquid crystal display device which is equivalent to that of other conventional liquid crystal display devices, it is necessary to increase the illuminance of the back light, which is the light source of the liquid crystal display device. However, the power consumption of a back light typically accounts for about ⅔ of the total power consumption of the liquid crystal display device. Therefore, the power consumption of the liquid crystal display device as a whole cannot be reduced in this way.
These problems have been addressed in the art by, for example, Japanese Laid-Open Publication Nos. 4-125617, 4-295824, 6-289408 and 8-201830, which disclose methods in which the surrounding region of the display pixel area is provided with an electrode pattern. An electric signal having a DC component is externally applied to the electrode pattern to adsorb the ionic impurity which has been introduced into the liquid crystal layer onto the electrode pattern, so as to maintain the purity of the liquid crystal layer in the display pixel area.
However, such conventional methods in which an electrode pattern is provided in the surrounding region of the display pixel area have the following problems.
In Japanese Laid-Open Publication No. 4-125617, an ion adsorption electrode pattern is provided on an active matrix substrate having TFTs provided thereon, while the display electrode on a CF substrate is not provided in a position opposing the electrode pattern.
However, the interval between a region of the display pixel area in which the ion adsorption electrode pattern is provided and a region in which a sealing material is provided is as small as about 1 mm to 3 mm. In order to ensure that the display electrode on the CF substrate does not oppose the ion adsorption electrode pattern, this may be too small for methods which are typically employed in the prior art, i.e., methods in which display electrodes are patterned while directly masking the display electrode portions with a metal mask during the display electrode formation. Thus, it is necessary to pattern the display electrodes on the CF substrate with a photolithography technique, thereby increasing the number of production steps.
Moreover, when such an electrode pattern is provided on a typical liquid crystal display device, an interlayer insulating film is employed to electrically isolate the electrode pattern from the source or gate signal lines which cross the electrode pattern. However, an inorganic film of silicon nitride (SiN), or the like, which is typically employed for the interlayer insulating film is deposited by a CVD (chemical vapor deposition) method, and has a thickness of several hundreds of nanometers and a dielectric constant as high as 8. Therefore, depending upon the potential to be applied to the electrode pattern, the obtained display may be substantially affected by the capacitance at the intersection between the electrode pattern and the signal lines.
In addition, according to the drawings of Japanese Laid-Open Publication No. 4-125617, a protective film is provided on the electrode pattern. When a TFT production process is considered, the protective film needs to be deposited separately, thereby further increasing the number of production steps.
Japanese Laid-Open Publication No. 4-295824 discloses an arrangement in which an ion adsorption electrode pattern is provided between a display region and a sealing material. This conventional technique is directed primarily to duty drive type liquid crystal display devices. Therefore, the electrode pattern can be provided only in a direction parallel to segment lines and in a direction parallel to common lines. Signals are input to the segment lines and the common lines individually. However, in a liquid crystal display device with TFTs, in order to input signals other than the counter potential to the CF substrate, which corresponds to the substrate on which the common lines are provided, it is necessary to pattern the display electrodes on the CF substrate by a photolithography technique as in Japanese Laid-Open Publication No. 4 -125617.
Japanese Laid-Open Publication No. 8-201830 discloses a similar arrangement for liquid crystal display devices with TFTs. This arrangement also has a problem of increasing the number of production steps.
In Japanese Laid-Open Publication No. 6-289408, an ion adsorption electrode pattern can be formed from a conductive film which is also used to form the TFTS. Therefore, the problem of increasing the number of production steps, as needed in the above three patent publications, can be avoided. In the liquid crystal display device of Japanese Laid-Open Publication No. 6-289408, one or both of an alignment film or an overcoat film is removed above the electrode pattern so that an alternating voltage is applied to the electrode pattern and a DC potential is applied across the liquid crystal layer. Therefore, the region in which the electrode pattern is formed is different from the display pixel area. Thus, an asymmetric potential (=DC component) corresponding to the dielectric constant of the removed alignment film or overcoat film is generated and applied across the liquid crystal layer.
However, this arrangement presumes that an overcoat film is provided over the display electrodes in the display pixel area. Typically-employed liquid crystal display devices do not have any film other than the alignment film provided on the display electrodes. Therefore, this conventional technique differs from a liquid crystal display device of the present invention in terms of the basic structure.
Moreover, Japanese Laid-Open Publication No. 6-289408 states that the above-described effects can be obtained with a DC potential of 5 mV to 100 mV. However, in high definition type (XGA or SXGA type) liquid crystal display devices of which the diagonal screen size is about 10 inches or more have a DC potential difference in the display screen plane occurring due to a signal delay through signal lines and/or display electrodes on the CF substrate. A DC potential difference as large as 100 mV has been observed in such liquid crystal display devices. Therefore, it is believed that the conventional technique cannot improve the visible defects which occur due to ionic impurities as described above.
According to one aspect of this invention, a liquid crystal display device includes: a pair of substrates opposing each other: a liquid crystal layer interposed between the pair of substrates; a plurality of switching elements arranged in a matrix pattern on one of the pair of substrates: gate signal lines for supplying gate signals for driving the switching elements; source signal lines crossing the gate signal lines for supplying display signals to the switching elements: an interlayer insulating film provided on one of the pair of substrates over the gate signal lines and the source signal lines; and pixel electrodes provided over the gate signal lines and the source signal lines via the interlayer insulating film. The interlayer insulating film on one of the pair of substrates extends to a surrounding region of a display pixel area. An electrode pattern for adsorbing an ionic impurity is provided on the interlayer insulating film in the surrounding region.
In one embodiment of the invention, the pixel electrodes are provided to partially overlap at least one of the gate signal lines and the source signal lines.
In one embodiment of the invention, the pixel electrodes and the electrode pattern are made of a metal material having a reflective property.
In one embodiment of the invention, the electrode pattern is provided inward with respect to a sealing material with which the pair of substrates are attached together.
In one embodiment of the invention, the electrode pattern is covered with an alignment film.
In one embodiment of the invention, an electric signal having a DC potential is input to the electrode pattern.
In one embodiment of the invention, an electric signal which is input to the electrode pattern is supplied from at least one of a power supply for a source driving circuit and a power supply for a gate driving circuit.
In one embodiment of the invention, the electrode pattern is divided into a plurality of segments, and an electric signal is individually input to each of the segments.
In one embodiment of the invention, the display pixel area has a generally rectangular shape. The pair of substrates are arranged so that a rubbing direction of one of the substrates which is represented by a first arrow crosses a rubbing direction of the other one of the substrates which is represented by a second arrow, the first and second arrow each extending from its tail to its head. The electrode pattern extends only along three sides of the display pixel area, including a first side interposed between the head of the first arrow and the head of the second arrow, and second and third sides which respectively extend from opposite ends of the first side.
In one embodiment of the invention, the pair of substrates are arranged so that a rubbing direction of one of the substrates which is represented by a first arrow crosses a rubbing direction of the other one of the substrates which is represented by a second arrow, the first and second arrow each extending from its tail to its head. The electrode pattern extends only along one side of the display pixel area interposed between the head of the first arrow and the head of the second arrow.
In one embodiment of the invention, the interlayer insulating film is made of an organic material.
In one embodiment of the invention, the liquid crystal display device includes a generally rectangular display pixel area. A rubbing direction of at least one of the substrates is represented by an arrow pointing to a corner of the generally rectangular display pixel area. The electrode pattern extends along two sides of the generally rectangular display pixel area which are connected together by the corner that is pointed to by the arrow.
In one embodiment of the invention, the liquid crystal display device includes a generally rectangular display pixel area. A rubbing direction of one of the substrates is represented by a first arrow pointing to a first corner of the generally rectangular display pixel area, and a rubbing direction of the other one of the substrates is represented by a second arrow pointing to a second corner of the generally rectangular display pixel area. The electrode pattern extends along a first pair of sides which are connected together by the first corner and along a second pair of sides which are connected together by the second corner, wherein the first pair of sides and the second pair of sides may share one side with each other.
In one embodiment of the invention, the electrode pattern is formed simultaneously with the pixel electrodes.
The functions of the present invention will now be described.
As described above, the present invention provides a liquid crystal display device in which pixel electrodes are provided over gate signal lines and source signal lines via an interposing interlayer insulating film, wherein the interlayer insulating film extends to a surrounding region of a display pixel area (As used herein, the term xe2x80x9csurrounding region of a display pixel areaxe2x80x9d refers to a region which surrounds the display pixel area and is outside the display pixel area) on which an electrode pattern for adsorbing an ionic impurity is provided. The electrode pattern can be formed simultaneously with the pixel electrode from the same material, thereby avoiding an increase in the number of production steps. Moreover, since an alignment film is provided on the electrode pattern, it is not necessary to separately provide a protective film, thereby eliminating the step of forming a protective film as in the prior art. Furthermore, since the counter electrode on the counter substrate (CF substrate) can be provided on the electrode pattern, it is not necessary to pattern the counter electrode on the CF substrate by a photolithography technique, or the like.
The pixel electrodes may be provided to partially overlap at least one of the gate signal lines and the source signal lines. The liquid crystal display device of the present invention may be a reflective liquid crystal display device in which the pixel electrodes and the electrode pattern are made of a metal material having a reflective property.
The interlayer insulating film may be made of an organic material. In such a case, it is possible to reduce the capacitance at each of intersections between the electrode pattern and the signal lines. As described in Japanese Laid-Open Publication No. 9-96837, an acrylic resin, for example, has a dielectric constant of 3.7 and can be deposited to a thickness of 1.5 xcexcm to 5 xcexcm using a spin coating method. Therefore, the capacitance at each of the intersections can be ⅙ to {fraction (1/22)} of that which would result when using a conventional insulating film made of silicon nitride, thereby reducing the influence on the display to a level such that the influence cannot be observed by the viewer.
An ionic impurity can be adsorbed onto the surf ace of the ionic impurity adsorbing electrode pattern by inputting a DC potential having the polarity opposite to that of the ionic impurity to the electrode pattern, thereby preventing the display quality from lowering due to the ionic impurity, while improving the reliability, as will be described below in Embodiments 1-3.
The electric signal is input to the ionic impurity adsorbing electrode pattern only to provide a potential difference across the liquid crystal layer and does not substantially flow as an electric current. Therefore, the electric signal can be supplied to the electrode pattern by using: a DC power supply for driving ICs, or the like, of driver circuits; a DC power supply for supplying a xc2x1potential for gate signals; a power supply for supplying rectangular wave signals such as source signals and common signals; and the like, which are used in the existing liquid crystal display devices.
By covering the ionic impurity adsorbing electrode pattern with an alignment film, an electrically attracted ionic impurity can be adsorbed onto the alignment film itself. Moreover, the alignment film can also function as an insulating film for preventing leakage between the electrode pattern and the counter electrodes on the counter substrate (CF substrate).
The ionic impurity adsorbing electrode pattern may be divided into a plurality of segments, and an electric signal may be individually input to each of the segments, as will be described below in Embodiment 3. Also in this way, it is possible to prevent visible defects due to an ionic impurity, while obtaining a good display.
Usually, contrast reductions are significant only along particular side/sides of the display pixel area. Therefore, the ionic impurity adsorbing electrode pattern may be provided only along these sides, as in Embodiment 2 to be described below.
The liquid crystal display device of the present invention may be a reflective liquid crystal display device in which the pixel electrodes and the ion adsorption electrode pattern are made of a reflective metal material.
Thus, the invention described herein makes possible the advantages of providing a liquid crystal display device which is capable of avoiding visible defects occurring due to an ionic impurity which has been introduced into the liquid crystal layer, whose display is not influenced by input signals to the electrode pattern, which can be produced without increasing the number of production steps, and in which it is not necessary to separately provide a source of signal input for the electrode pattern.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.